Apart from the word for "forehead" being diagnostically different in the various dialects of Klingon (as pointed out by @JeffZeitlin), the only other canonical information I know of is that in some dialects, <D> and <b> /ɖ, b/ are pronounced as /ɳ, m/ (<N> and <M>—though I don't remember if those transliterations are canonical, or invented by me for the Klingon Hamlet).
EDIT: More about dialects from http://klingonska.org/piqad/, citing Klingon for the Galactic Traveller. (I was right that I made up <M>, and <N> is not canonical.)
The Tak’ev Dialect
Dialect spoken in the Tak’ev (taq’ev) region. It is one of the larger
minority dialects and has a much greater number of speakers that the
Krotmag dialect. It sounds like blend of Krotmag an Standard Klingon,
and is the one dialect (that we know of) which most closely resembles
Standard Klingon. It is only briefly described in canon.
- Nasal vowels
- Pronounces b as mb
- Pronounces D as ND
The Krotmag Dialect
Dialect spoken in the Krotmag (Qotmagh) region. This dialect has
fewer speakers than the Tak’ev dialect, but is well known and easily
recognized by other Klingons. Some of the dialects peculiarities have
even influenced Standard Klingon.
- Nasal vowels
- Pronounces b as m (as in English mime)
- Pronounces D as N
- Often uses extra words in noun phrases, originally to differentiate between words with b and m (e.g. ’uS qam leg foot, nach qam head face), but now prevalent even when no disambiguation is needed (e.g. NeS ghop arm hand, nach ghIch head nose, qorNu’ tuq family house and even yan ’etlh or ’etlh yan sword sword)
- Often adds short extra sentences to disambiguate between verbs with b and m (e.g. mImoH. yIjotchoH. You’re impatient. Calm down!, mImoH. ’oy’ mInNu’wIj. You’re ugly. My eyes ache.)
The Morskan Dialect
Dialect spoken on the conquered Klingon world Morska. We’re treated to
brief bout of the Morskan dialect of Klingon in ST6 when Enterprise
encounters a Klingon listening post. (The Klingon spelling of the word
“Morska” is unknown – the only canon occurrence is in spoken form, in
the Morskan dialect, by that guard in ST6).
The Morskan dialect is characterized by the following speech patterns.
- Pronounces tlh as ghl at the beginning of syllables, and as ts (as in English cats) at the end
- Pronounces H as h (as in English hat) at beginning of syllables, and not pronounced at all at end
- Pronounces Q as Standard Klingon H
- Usually drops -’e’ from the final noun in “to
be” phrases (except when the subject of such phrases are emphasised)